Nitric oxide (NO), has been shown to have profound effects on the function of leukocytes and in the overall host immune response to infection, sepsis and septic shock. NO is synthesized by three different isoforms of NO synthase (NOS). Two of these isoforms, NOS1 and NOS3 are constitutively expressed, while the third, NOS2, is an inducible (by toxins and cytokines) form of the enzyme. Several lines of evidence suggest that NOS1 may have a very important role in the pathophysiology of sepsis. The enzyme is constitutively expressed not only in neuronal cells in the brain and spinal cord, but also in the microvasculature and epithelium of the gastrointestinal tract and kidney, bronchial epithelium, myocytes of skeletal muscle, mast cells in skin, and neutrophils. Under baseline conditions and during sterile peritonitis, mice congenitally lacking NOS1 (NOS1 knock out, NOS1-KO) have increased leukocyte rolling and adhesion to the endothelium of postcapillary venules and increased leukocyte migration into the peritoneal cavity. We are investigating the effects of NOS1 on extravascular neutrophil recruitment, bacterial clearance, and inflammatory tissue injury during polymicrobial peritonitis, sepsis, and septic shock. At baseline, NOS1-KO animals have lower white blood cell, neutrophil, platelets and hemoglobin counts. We have shown that although NOS1-KO animals have reported increased migration of neutrophils into the peritoneum, in the setting of live bacterial peritonitis, genetic deficiency of NOS1 is detrimental and increases mortality. In addition, we found that 7-nitroindazole, a specific inhibitor of NOS1, had opposite effects in wild type animals compared to NOS1-KO. The mechanisms of this increased mortality in NOS1-KO animals are yet unknown but it appears that NOS1-KO have higher systemic bacterial counts in blood compared to wild type animals suggesting that animals deficient in NOS1 may have decreased bacterial clearance. We are now studying the mechanism whereby NOS1 impacts on survival and bacterial clearance during sepsis and septic shock. Specifically, we are studying the function of neutrophils fom NOS1-KO animals and their response to endotoxin challenges.